Kits used for guiding of ballistic and direct aiming projectiles to their target are known in the art. Such kits are typically of relatively high precision and are very expensive or of relatively very low precision and of lower cost. Use of a projectile guiding kit is suitable where ‘statistic firing’ (that is, where a large number of ammunition units with low accuracy is fired towards one target in order to hit it) is expected to improve the ratio of circular error probability (CEP) to cost (number of fired ammunition units). In order to enable this improvement, at least one of the figures—cost of guiding kit and CEP—needs to improve, that is the cost of a guiding kit needs to get lower and/or the CEP of a projectile equipped with a guiding kit needs to improve, so that the product of both prove improved efficiency. While expensive guiding kits enable efficient guiding of a projectile, where less kinetic energy of the projectile is dissipated due to the guiding maneuvering, low-cost guiding kits known in the art typically dissipate a lot of the kinetic energy of the projectile and as a result shorten its range and lower its final speed, which in turn lower its accuracy. Typically, the cost of a guiding kit for a projectile is derived mainly from the number of control variables it consists.
One control variable is the amount of resistance to rotation provided between the main body of the projectile and the projectile guiding kit axially connected to it, typically in front of it. Most of the guiding kits consist of an alternator disposed between the main body of the projectile and its guiding kit. One or more fins that are installed on outer skin of the guiding kit frontal member may cause this member to rotate in a speed that is different from the rotation speed of the projectile and typically lower than that rotation speed. The difference in rotation speeds can be utilized to rotate a stator and rotor of an alternator (or a similar electricity producing device). The alternator may be loaded with a controllable electrical load. Changes in the amount of electrical load applied to the alternator will change the amount of rotational resistance produced by that alternator.
Additional control variables may be embodied by one or more fins (or canard wings), the angle of attack of which may be controlled to achieve various control targets such as stabilizing the rotation of the nose of the guiding kit with respect to an external reference frame, such as the horizon; providing lift and/or turn aerodynamic forces in order to guide the projectile to its target, etc. Each fin whose angle of attack needs to be controlled seriously raises the cost of the guiding kit, because a controllable actuator needs to be provided and to be attached between the guiding kit and the respective fin and to control its angle of attack at every moment of the flight.
U.S. Pat. No. 6,981,672 to Clancy et al. discloses a guiding kit with two pairs of aerodynamic surfaces (or canard fins) both having fixed angles of attack. The angles of attack of one pair of fins are selected to spin the nose of the guiding kit in a direction opposite to the direction of spin of the projectile. The angles of attack of the second pair of fins are selected so that, when the nose spins, their net effect on the projectile flight is null, and when the projectile nose does not spin with respect to an external reference frame this pair of fins induces a lateral force and moment on the projectile flight direction, in a direction that is substantially perpendicular to the plane of these fins. This guiding kit utilizes only one control variable—the amount of rotational resistance provided by a spin control coupling (e.g., an alternator). This guiding device needs to provide a large anti-spin power at the beginning of the trajectory due to the high aerodynamic forces induced on the aerodynamic surfaces at the very high flight speeds at the beginning of the trajectory. The high anti-rotational power causes a lot of energy dissipation (e.g., by heat dissipated at the electrical load). Further, the pre-set angles of attack, which practically need to be adjusted to some average flight speed, produce high aerodynamic drag during the first portion of the trajectory, which also causes energy dissipation additionally to that of the anti-spin energy dissipation. As a result, at the beginning of the trajectory a lot of energy is dissipated only because the fins have a fixed pre-set angle of attack that is adjusted for lower speeds. The dissipated energy is consumed from the kinetic energy of the projectile and/or from its rotational energy, which in both cases is a disadvantage because it causes the shortening of trajectory of the projectile and the lessening of the projectile's longitudinal stability. When the projectile approaches the highest point of the trajectory (typically between 1000 and 15,000 meters), the aerodynamic effect of the fins reaches its lowest aerodynamic efficiency point due to the drop in the projectile's speed and in air density. For example, a projectile for a 20 km range may reach an initial speed of 700 m/s when leaving the cannon, may reach a maximum flight height of 6000 m above the ground where the speed will be about 280 m/s and the speed when the projectile is at the end of the trajectory may be about 350 m/s. As may be seen, the flight speed of the projectile changes by more than 60% during its flight, and the air density may change by over 50% from low level density to the top of the trajectory. For a projectile adapted to reach a range of 40 km, the range of change in the flight parameters may be even higher. As a result, the efficiency of a guiding kit with aerodynamic surfaces set in fixed angles of attack drops even lower while the total energy loss grows higher as the range of the projectile extends. The requirement for a higher lift capability in order to provide better control capability, and the requirement to limit the fins' angle of attack in order to lower the drag at launch are conflicting and, therefore, force the designer to choose between them, causing the requirement for higher controllability to be compromised.
Guiding kits for projectile which are known in the art typically fail to prove the required improvement of the combination of the two features. Accordingly, there is a need for a low-cost, simple and accurate projectile guiding kit, or device, which is capable of adapting its performance to the changes in flight parameters along the flight trajectory.